1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary cell, particularly, to the improvement of a non-aqueous solvent.
2. Description of Prior Art
Small-sized and light electronic instruments have been realized by the progress of recent remarkable electronic technology. With this progress, it has been demanded to make a cell small and light and to make energy density high to the cell as a portable electric source.
Conventionally, aqueous cells including a lead, a nickel, a cadmium cell and so on are the main current of secondary cells for general use. Though these aqueous secondary cells are satisfactory in a cycle characteristic to some extent, the characteristic is not satisfactory in weight of the cell and the energy density.
On one hand, recently, a non-aqueous electrolyte secondary cell that uses lithium or lithium alloy for a negative electrode has been popularly studied and developed. This cell has high energy density by using oxide compounds including Li, such as LiCoO.sub.2, as a positive electrode, and has superior characteristics of little self-discharging and light weight.
However, in the non-aqueous electrolyte secondary cell that uses Li or Li alloys for the negative electrode, with the increasing of a charging/discharging cycle, lithium proceeds up a dendrite crystal, it reaches the positive electrode and an inner short might occur. Further, as the dendrite crystal is being created, a problem arises that it is impossible to practically and rapidly charge and discharge. For this reason, it is hard to practically make the non-aqueous electrolyte secondary cell that uses Li or alloys of Li for the negative electrode.
In order to resolve these problems, the non-aqueous electrolyte secondary cell (the lithium ion secondary cell) of a so-called rocking chair type that uses layer compounds of oxides and carbons and so on, in which lithium ions are taken, for the negative material has been proposed. In such a non-aqueous electrolyte secondary cell, it is used for the reaction of the negative electrode that lithium is doped/dedoped between layers of these layer compounds. This occurs even if the charging/discharging cycle is increased, dendritic deposition is not realized and the good charging/discharging cycle is shown.
Though each kind of carbon material that can possibly be used for the negative material of the non-aqueous electrolyte secondary cell has been described, the carbon materials that have been made practical as the negative materials are graphitization retardant carbon materials, that is, low crystalline carbon materials obtained by performing thermal treatment on organic materials at comparatively low temperature. The non-aqueous electrolyte secondary cell that is formed when the negative electrode, including graphitization retardant carbon materials, is combined with an electrolyte, where propylene carbonate (PC) is the main solvent, has already been produced.
Further, recently, it has also been possible to use the kinds of graphites where crystal structure has been developed. In the case of these kinds of graphites, as PC used for the main solvent is resolved, it was a problem that the kinds of graphites are the negative electrode materials. However, the problem has been addressed by making ethylene carbonate (EC), which is very stable, as the main solvent, and it has then been possible to use them as the negative materials.
It is possible to obtain comparatively easily these kinds of scaly graphites, and they have been widely used as conductive agents for alkali cells and so on as usual. These kinds of graphites are highly crystalline and have high actual density compared to graphitization retardant carbon materials. Therefore, if the negative electrode is formed with these kinds of graphites, it is possible to obtain a high electrode charging characteristic and the energy density of the cell is advanced. Therefore, these kinds of graphites are the materials which are largely expected as the negative electrode material.